KIEV, Ukraine (CNN) -- International monitors investigating the Malaysia Airlines crash in eastern Ukraine said Friday the team was not given full access to the site and was greeted with hostility by armed men guarding the scene.
"It basically looks like the biggest crime scene in the world right now, guarded by a bunch of guys in uniform with heavy firepower who are quite inhospitable," Michael Bociurkiw, spokesman for the Organization for Security and Co-operation in Europe team, told CNN's Christiane Amanpour.
"And there didn't seem to be anyone really in control, for example. One of our top priorities was to find out what happened to the black boxes. No one was there to answer those questions."
The 21-member OSCE team arrived at the crash site near Torez in a remote section of eastern Ukraine that's controlled by pro-Russian militants who are battling the Ukraine government. The identity of the armed men at the site was not clear.
The United States says a surface-to-air missile, possibly fired by the militants, took down Malaysia Airlines Flight 17 on Thursday as the plane traveled from Amsterdam to Kuala Lumpur in Malaysia. All 298 people on board died.
Lack of access worries U.S. officials, including Ben Rhodes, deputy national security adviser for strategic communications, who tweeted: "Monitors should be able to access the crash site of MH17. US is deeply concerned by reports that separatists are denying access."
Bociurkiw said the group of monitors only got to stay about 75 minutes on the scene. Missiles could be heard in the distance, which made the monitors uneasy, he said.
Bociurkiw said he doesn't know of any bodies moved from the site, but warned that the integrity of the scene could be compromised. The crash site is in a difficult-to-access area in the country with no electricity.
"The perimeter is not secure whatsoever," he said. "They seemed to have put some tape up where we were standing. But, otherwise, it's very easy for anyone, really, to walk in there and tamper with evidence or debris. So a lot of work needs to be done. A lot of professional work, very very quickly."
Noah Sneider, a freelance journalist who visited the site, told CNN that debris was scattered over about 5 acres on a farm field. He reported seeing some victims still in their airliner seats wearing headphones.
"Especially prominent were headphones and tropical shorts, swimsuits, sandals, things people would take to a beach vacation," Sneider said.
The FBI is sending two investigators to work on the case, a U.S. law enforcement official said, but the Ukraine government will be in charge of the investigation.
Obama puts focus on Russia
Russia likely bears some of the responsibility for the apparent downing of Flight 17, President Barack Obama indicated Friday.
In the administration's strongest words yet on the downing of the jet, which left 298 people dead, Obama said rebel fighters couldn't have operated the surface-to-air missile believed responsible for the shootdown "without sophisticated equipment and sophisticated training, and that is coming from Russia."
He and other U.S. officials stopped short of publicly placing the responsibility on Russia, which has denied any involvement in the destruction of the jetliner.
But a senior defense official told CNN that the "working theory" among U.S. intelligence analysts is that the Russian military supplied the Buk missile system to rebel fighters inside Ukraine.
U.S. officials believe the plane was "likely downed by a surface-to-air missile ... operated from a separatist-held location in eastern Ukraine," the U.S. ambassador to the United Nations, Samantha Power, told an emergency session of the U.N. Security Council.
In his remarks to reporters, Obama said that he did not want to get ahead of the facts of who may have been directly involved in the airliner's destruction. But he said the United States would work hard to hold accountable those responsible for it.
At least one American on board
Obama confirmed that at least one U.S. citizen was aboard the plane; Quinn Lucas Schansman was a student at International Business School Hogeschool van Amsterdam, according to his Facebook page. A majority of the passengers (at least 173) were Dutch.
"No one can deny the truth that is revealed in the awful images that we all have seen, and the eyes of the world are on eastern Ukraine, and we are going to make sure that the truth is out," Obama said.
He called for an immediate cease-fire in the region and for a "credible international investigation" into what happened.
His comments echoed earlier statements by the U.S. ambassador to the United Nations, Samantha Power, who pointed the finger Friday at pro-Russian rebels.
Power told an emergency meeting of the U.N. Security Council that the plane was "likely downed by a surface-to-air missile ... operated from a separatist-held location in eastern Ukraine." If pro-Russian separatists are responsible for shooting down the plane with a missile, investigators can't rule out the possibility that Russia offered help to operate the system, she said.
Power also said Russia should take steps to cool tensions in Ukraine.
"Russia can end this war," she said. "Russia must end this war.
Among the evidence cited by U.S. officials and others for their conclusions was an audio recording released by Ukrainian intelligence officials which purportedly feature pro-Russian rebels and Russian military officers discussing a surface-to-air strike and the crash of a civilian jetliner.
"How are things going there," a man identified as a Russian intelligence agent asks.
"Well, we are 100% sure that it was a civilian plane," a man identified as a pro-Russian fighter responds.
"Are there a lot of people?" the Russian officer asks.
The rebel fighter then utters an obscenity and says, "The debris was falling straight into the yards."
CNN cannot confirm the authenticity of this audio, or other similar recordings.
Also, in a news conference Friday, the chief of Ukraine's security service, Valentyn Nalyvaichenko, said the Buk missile system that shot down the airliner crossed the border from Russia only "right before" the attack. He didn't say how investigators know that, however
Ukraine's Interfax news agency reported claims by an adviser to Ukrainian Minister of Internal Affairs Anton Geraschenko that the launcher, as well as the flight data recorders from MH17, were handed over to Russian agents across the border at a checkpoint in the Luhansk area overnight.
A senior Ukrainian official who spoke to CNN also accused Russia of carrying out a cover-up of its role in the shoot-down.
He cited video showing a Buk launcher being moved towards Russia overnight.
CNN could not independently confirm the claims.
Tensions have been high between Ukraine and Russia since street protests forced former pro-Moscow President Viktor Yanukovych from power in February. Russia subsequently annexed Ukraine's southeastern Crimea region, and a pro-Russian separatist rebellion has been raging in Ukraine's eastern Luhansk and Donetsk regions.
Ukraine's government has accused Russia of allowing weapons and military equipment, including tanks, to cross the border illegally into the hands of pro-Russian rebels.
While Ukrainian officials implicated pro-Russian fighters and their Russian backers for the jetliner's downing, Moscow argued Ukraine was to blame.
"With regard to the claims raised by Kiev, that it was almost us who did it: In fact I haven't heard any truthful statements from Kiev over the past few months," Russian Foreign Minister Sergey Lavrov said in an exclusive interview with the state-run Russia 24 TV channel.
European Union leaders agreed this week to expand sanctions against individuals and entities in response to Russia's actions in Ukraine, with details to be decided by the end of the month. Expanded U.S. sanctions were also announced in Washington.
The Ukrainian Ministry of Infrastructure announced Friday that the airspace over Donetsk, Luhansk and part of Kharkiv where rebels are operating had been closed indefinitely.
Malaysian Transport Minister Liow Tiong Lai defended the routing of the Malaysia Airlines plane over the region, saying other carriers were sending their aircraft through the same airspace.
Three months ago, the U.S. Federal Aviation Administration prohibited U.S. airlines from flying in areas some way south of where Flight 17 crashed Thursday. Thursday night, the FAA expanded the flight restrictions to all of eastern Ukraine.
Thursday's crash marks the second time this year that Malaysia Airlines has faced an incident involving a downed plane.
In March, Malaysia Airlines Flight 370 disappeared with 239 people on board. Searchers have found no trace of the Boeing 777 or its passengers despite extensive search efforts.
Flight 370 probably flew into the southern Indian Ocean on autopilot with an unresponsive crew, Australian authorities said last month. A new underwater search is expected to begin in August.